Mechanistic Investigation of Oxidative Methanol Carbonylation with Mixed Cu(I)/Cu(II)–N-Methylimidazole Catalysts

Copper complex catalysts exhibit excellent activity for the oxidative carbonylation of methanol to produce dimethyl carbonate (DMC). However, the reaction mechanism over Cu(I) or Cu(II) complex catalysts is not fully understood. In this study, homogeneous CuCl/CuCl₂–NMI-m/n (N = N-methylimidazole; m/n = molar ratio of Cu(I) and Cu(II)) catalysts with organic ligand coordination are designed for oxidative carbonylation. The optimized CuCl/CuCl₂–NMI-0.3/0.7 catalyst showed a TOF as high as 3.4 h^–1 and a DMC selectivity of 67.2% based on oxygen, which were superior to those of CuCl–NMI or CuCl₂–NMI. Our experiments suggested that the high activity and selectivity were assigned to the synergistic effect between Cu(I) and Cu(II), in which Cu(I) and Cu(II) are mainly responsible for the rapid generation of copper carbonyl and copper methoxy intermediates, respectively. This synergistic effect not only enhances the reaction activity but also ensures high DMC selectivity.